1. Technical Field
The present invention relates to a process and apparatus for electrolytically generating strong solutions of halogen oxyacids from the corresponding alkali metal salt. The present invention will be particularly described with reference to generating a chloric acid (HClO.sub.3) solution of high normality from sodium chlorate (NaClO.sub.3). However, it will be apparent to those skilled in the art that the present invention is also applicable to the generation of other oxyacids, for instance perchloric acid (HClO.sub.4) from sodium perchlorate (NaClO.sub.4).
2. Description of the Prior Art
One problem with halogen oxyacids such as chloric acid is that they are unstable and subject to decomposition, particularly at elevated temperatures. This prevents them from being easily stored and shipped requiring that they be made at a point of use rather than on a large industrial scale. Present commercial methods for generating chloric acid on site acidify sodium chlorate with sulfuric acid. This produces an impure product stream containing sodium sulfate which has to be removed, and which is of little value as a by-product.
U.S. Pat. No. 4,798,715, assigned to the assignee of the present application discloses the production of chloric acid from sodium chlorate using an ion exchange resin. Chlorine dioxide is then manufactured by reducing the chloric acid in an electrochemical cell. It is disclosed in the patent that the chloric acid feed to the electrochemical cell can have a normality of about 0.5 up to about 4.5. However, it is desirable to feed chloric acid to the electrochemical cell at a relatively high normality, for instance, above about 1.5, in order to obtain reduction of the chloric acid to chlorine dioxide at optimum efficiency.
U.S. Pat. No. 3,810,969 also discloses the manufacture of chloric acid by reacting an alkali metal chlorate with a stoichiometric excess of a cation exchange resin. One problem with the use of a cation exchange resin is that such resins have a relatively short lifetime, increasing the cost of manufacture of chloric acid.
It is known to produce acids using an electrolytic cell. U.S. Pat. No. 4,115,217 discloses the use of a three-compartment electrolytic cell for the preparation of sodium chlorite (NaClO.sub.2) from sodium chlorate (NaClO.sub.3), sulfuric acid, and sulfur dioxide. A product of the process of this patent is enriched sulfuric acid (H.sub.2 SO.sub.4) instead of chloric acid. The process comprises reacting the sodium chlorate in a reactor with the sulfur dioxide to produce a residual solution of sodium sulfate and sulfuric acid. Chlorine dioxide (ClO.sub.2) is also formed in the reactor and is removed in an inert gas stream. The residual solution containing sodium sulfate and sulfuric acid is fed into the middle compartment of the electrolytic cell. The middle compartment is defined on one side by an anion selective membrane and on the opposite side by a cation selective membrane. The end compartments of the cell are an anode compartment separated from the middle compartment by the anion selective membrane and a cathode compartment separated from the middle compartment by the cation selective membrane. When a voltage is applied to the cell, sulfate ions migrate from the middle compartment through the anion selective membrane into the anode compartment. At the anode, water is decomposed with the evolution of oxygen and generation of hydrogen ions which react with the migrated sulfate ions to form sulfuric acid. The chlorine dioxide formed in the reactor is fed into the catholyte of the three-compartment cell and is reduced to chlorite ions (ClO.sub.2.sup.-) at the cathode. The cations in the middle compartment, mainly sodium and hydrogen ions, migrate through the cation selective membrane. The sodium ions react with the chlorite ions formed at the cathode to form sodium chlorite which is precipitated in the cathode compartment when saturation is reached.
Problems arise, however, when an electrolytic cell such as that disclosed in U.S. Pat. No. 4,115,217 is attempted to be used in the manufacture of a strong halogen oxyacid such as chloric acid. For one, such cells are known to generate substantial amounts of heat because of solution and separator resistance, which can lead to acid decomposition. In addition, the halogen oxyacids at high temperature are highly corrosive preventing many materials conventionally employed in electrolytic cells from being used in association with the oxyacids.
U.S. Pat. No. 3,222,267 also describes a three-compartment cell. An electrolytic solution is electrolyzed in such a manner as to produce salt-free product hydroxide and the corresponding acid salt of sodium bisulfate. By way of example, a 10% sodium sulfate solution was introduced into a center feed compartment. The flow rate and pressure of the solution is sufficient for the solution to percolate through a porous diaphragm into an anode compartment. The flow rate and pressure also prevents back migration or diffusion of protons toward the cell cathode. Water is introduced into the cathode compartment. Electrolysis in the cell produces a 2N sodium hydroxide catholyte effluent and a 0.075N acid anode effluent. As in U.S. Pat. No. 4,115,217, the compositions of the solutions, were not such that decomposition of the effluents, or corrosion of materials conventionally used in such a cell, were a problem.
A disclosure similar to that of U.S. Pat. No. 3,222,267 is contained in U.S. Pat. No. 3,523,755.
U.S. Pat. No. 2,829,095 discloses a process for the production of acidic solutions in an electrolytic cell using a plurality of anion exchange and cation exchange membranes. There is no disclosure concerning the production of halogen oxyacids.
U.S. Pat. No. 4,504,373 discloses a three-compartment electrodialytic cell and feeding alkali metal sulfate values to the cell.
U.S. Pat. No. 4,740,281 discloses supplying a salt and acid to one compartment of an electrodialysis apparatus and a liquid containing water to a second compartment of the apparatus. The process is for regenerating acids from stainless steel pickling baths.